True. But it's only a few interfaces. 25 KB. And probably it is possible to just include the single interface you need directly into Smack, if you worry about including the complete API.
On the other hand, you have your own interface which would be unecessarily included in normal Java SE environment.
It doesn't really matter for both sides, because nobody cares for a few KBs on their system.
Performance wise, I don't see your point. You would still use the Android parser. It's just accessed through another interface. There's no logic in the adapter class, just method delegation, and that's no performance risk.